Last December, a federal appeals court agreed to a request by environmental groups to temporarily block the changes to the EPA's "new source review" rules. The court said the rules could not take effect until the lawsuit challenging their legality was finished.

Emissions from older coal-fired power plants and refineries can aggravate asthma, chronic bronchitis and pneumonia.

The EPA said as part of reconsidering the changes, it would allow industry, environmental groups and others to submit suggestions during the next two months. The agency will specifically review the threshold it set for replacement costs in certain industrial units and the procedure for how states should comply with changes in the federal air pollution rules.

Under the EPA's planned rules, a facility, such as a power plant, could replace equipment without installing pollution controls as long as the cost of the replacement did not exceed 20 percent of the cost of the plant.

Michael Leavitt, EPA administrator, told reporters he expected the appeals court to eventually uphold the agency's new source review rule.

"We do feel optimistic that the rule we put forward will be upheld," Leavitt said at a news conference about environmental issues at the Interior Department. "We did accept their request for reconsideration as a matter of process, and if there is any new information they have we'll be willing to receive it."

When Congress wrote the new source review provision of the Clean Air Act in 1977, it assumed most of the aging coal-fired plants would be gradually replaced with new ones. Congress exempted plants operating at the time from stricter pollution controls, unless they launched a major renovation or expansion.